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WO2010139995A1 - Dispositif à cristaux liquides comprenant un matériau à cristaux liquides nématiques chiraux ayant un agencement hélicoïdal - Google Patents

Dispositif à cristaux liquides comprenant un matériau à cristaux liquides nématiques chiraux ayant un agencement hélicoïdal Download PDF

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Publication number
WO2010139995A1
WO2010139995A1 PCT/GB2010/050929 GB2010050929W WO2010139995A1 WO 2010139995 A1 WO2010139995 A1 WO 2010139995A1 GB 2010050929 W GB2010050929 W GB 2010050929W WO 2010139995 A1 WO2010139995 A1 WO 2010139995A1
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WIPO (PCT)
Prior art keywords
liquid crystal
electric field
chiral nematic
helical
light
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PCT/GB2010/050929
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English (en)
Inventor
Su Soek Choi
Flynn Castles
Stephen Morris
Damian Gardiner
Harry Coles
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Cambridge Enterprise Ltd
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Cambridge Enterprise Ltd
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Priority to CN2010800342348A priority Critical patent/CN102460290A/zh
Priority to US13/322,834 priority patent/US20120140133A1/en
Priority to EP10725843A priority patent/EP2438485A1/fr
Publication of WO2010139995A1 publication Critical patent/WO2010139995A1/fr
Anticipated expiration legal-status Critical
Ceased legal-status Critical Current

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    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
    • G02F1/13—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells
    • G02F1/133—Constructional arrangements; Operation of liquid crystal cells; Circuit arrangements
    • G02F1/1333—Constructional arrangements; Manufacturing methods
    • G02F1/1343—Electrodes
    • G02F1/134309—Electrodes characterised by their geometrical arrangement
    • G02F1/134363—Electrodes characterised by their geometrical arrangement for applying an electric field parallel to the substrate, i.e. in-plane switching [IPS]
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
    • G02F1/13—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells
    • G02F1/137—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells characterised by the electro-optical or magneto-optical effect, e.g. field-induced phase transition, orientation effect, guest-host interaction or dynamic scattering
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
    • G02F1/13—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells
    • G02F1/133—Constructional arrangements; Operation of liquid crystal cells; Circuit arrangements
    • G02F1/1333—Constructional arrangements; Manufacturing methods
    • G02F1/1343—Electrodes
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
    • G02F1/13—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells
    • G02F1/137—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells characterised by the electro-optical or magneto-optical effect, e.g. field-induced phase transition, orientation effect, guest-host interaction or dynamic scattering
    • G02F1/13718—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells characterised by the electro-optical or magneto-optical effect, e.g. field-induced phase transition, orientation effect, guest-host interaction or dynamic scattering based on a change of the texture state of a cholesteric liquid crystal
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
    • G02F1/13—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells
    • G02F1/133—Constructional arrangements; Operation of liquid crystal cells; Circuit arrangements
    • G02F1/1333—Constructional arrangements; Manufacturing methods
    • G02F1/1343—Electrodes
    • G02F1/134309—Electrodes characterised by their geometrical arrangement
    • G02F1/134381—Hybrid switching mode, i.e. for applying an electric field with components parallel and orthogonal to the substrates
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
    • G02F1/13—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells
    • G02F1/137—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells characterised by the electro-optical or magneto-optical effect, e.g. field-induced phase transition, orientation effect, guest-host interaction or dynamic scattering
    • G02F1/13775—Polymer-stabilized liquid crystal layers

Definitions

  • This invention generally relates to, inter alia, a liquid crystal device for controlling transmission of polarised light, a display device having a plurality of the liquid crystal devices, an optical waveguide device comprising the liquid crystal device, a variable optical attenuator (VOA) comprising the liquid crystal device, an optical switch or light shutter comprising the liquid crystal device, a laser comprising the liquid crystal device, a method of controlling outputting of light from a liquid crystal device, and a method of controlling transmission of polarised light, and to a further liquid crystal device.
  • VOA variable optical attenuator
  • LCD liquid crystal displays
  • the following aspects generally concern embodiments using LC having negative dielectric anisotropy.
  • a liquid crystal device for controlling transmission of polarised light, comprising: chiral nematic liquid crystal (LC) having a helical arrangement of liquid crystal molecules in the absence of an electric field; and at least two electrodes for applying an electric field having a component normal to the helical axis of the chiral nematic liquid crystal, wherein the chiral nematic liquid crystal has negative dielectric anisotropy.
  • LC chiral nematic liquid crystal
  • the rotation of the liquid crystal helical arrangement may due to dielectric coupling of the electric field and liquid crystal.
  • the coupling may be through the dielectric anisotropy rather than through flexoelectric coefficients.
  • the dielectric coupling may be found in combination with flexoelectric coupling.
  • flexoelectric coefficients may in some embodiments be optimised, e.g., minimised or maximised, to allow flexoelectric coupling to be used in combination with dielectric coupling. (This paragraph applies to every aspect described herein, including those using negative dielectric anisotropy and those using positive dielectric anisotropy).
  • Embodiments of the aspect may have a combination of at least one or all of short- pitch, uniform standing helix (USH) with negative dielectric anisotropy and in-plane electrodes.
  • short pitch may be taken as meaning having pitch shorter, preferably substantially shorter, than the wavelength of the controlled light or, more specifically, shorter than visible wavelengths, i.e., less than 380nm.
  • the device may have a light source for sourcing the light to be controlled.
  • Embodiments of such a liquid crystal device may be configured to control transmission of unpolarised or polarised light.
  • Embodiments may have any/use one or more of the following features: polymer stabilisation of the LC molecular arrangements; dielectric coupling effect to increase a switching effect of the device/method; at least one polariser, the LC preferably disposed between crossed polarisers; the LC being short pitch LC; an optic axis induced by an applied electric field, preferably between crossed polarisers, to give the switching effect; and uniform tilt of the optic axis to be in-plane (e.g., parallel to the plane of the electrodes), preferably between crossed polarisers, to give the switching effect; non-random optic axis when the electric field is applied.
  • a controlled amount may be a degree/amount of transmission, e.g., a proportion of the light that is passed or attenuated/absorbed by the device. Additionally or alternatively, the control may control the direction of outputting of light from the device.
  • the controlled light may be, for example, circularly or elliptically polarised. This may be determined by the helical structure.
  • the liquid crystal may be a material, e.g., liquid or semi-solid.
  • the helical arrangement may be of molecular orientations, e.g., of long axes of the liquid crystal molecules.
  • the direction normal to the helical axis of the LC is perpendicular to the axis of the above helical arrangement that exists in the absence of any applied electric field, i.e., the zero-field helical axis.
  • the electric field component may be an electric field local to at least a portion (e.g., sub-region) of the LC, for example an electric field in a region of a curved electric field distribution shown in Fig. 1 where the electric field is in-plane; in other scenarios the component may be a resolved vector component of a curved or uniform electric field, the field at least local to the component, the local electric field being non-parallel to the zero-field helical axis).
  • the LC may be comprised in a composition having a low concentration of reactive mesogen (e.g., Merck RM-257) or polymer.
  • the electric field application may involve applying a voltage such that a potential difference exists between the two electrodes, for example using in-plane electrodes of the device in the form of an in-plane switching device.
  • the polymer which may by used for LC stabilisation in any one of the aspects described herein (including those using negative dielectric anisotropy and those using positive dielectric anisotropy) may be a diacrylate structure that is photophoymerised using a UV light source with the addition of a low concentration of photoinitiator. This forms a polymer network.
  • the negative dielectric anisotropy may be obtained by means of a negative dielectric constant, and/or particularly at frequencies used/found in the driving signal for applying the electric field to the electrodes.
  • a negative dielectric constant as such is different to a negative dielectric anisotropy, which generally just means the dielectric constant parallel is less than the dielectric constant perpendicular to the director.
  • the property of having a negative dielectric anisotropy can vary with frequency.
  • the LC composition has a negative dielectric anisotropy at the frequency used in the drive signal.
  • the above liquid crystal device configured such that said chiral nematic liquid crystal molecules are helically arranged in the presence of said electric field, the helical axis (of said arrangement in said presence of said field) being aligned to the electric field that exists locally to said molecules.
  • the helical arrangement is rotated when the electric field is applied between said electrodes, the rotation being reversible.
  • the helical arrangement of LC molecules then existing even in the presence of the electric field is aligned to the orientation of the electric field local to the molecules of that arrangement.
  • a controllable degree of effective rotation of an LC helix may be achieved depending for example on the potential difference applied across the electrodes.
  • the helical axis thus reorients when an electric field is applied between the electrodes, preferably to lie in the plane of the said electrodes.
  • the liquid crystal (LC) director may lie along the direction of the electrodes, e.g., substantially in the plane of the in-plane switching device.
  • Such a state is an active, transmissive state. More particularly, a state of full alignment of the helical axis to the in-plane electrodes, i.e., in or parallel to the plane of the said electrodes, is preferably an active transmissive state, and/or may be one wherein the helical axis is parallel to the plane of the said electrodes or to the local electric field.
  • the electric field causing reorientation of the helical axis to lie in (e.g., parallel to) the plane of the electrodes may be viewed as causing an effective optic axis to be induced in-plane.
  • any reference herein to a rotated optic axis may more specifically be described as an induced optic axis, which may be at least partially aligned with the applied electric field.
  • the electrodes may be, for example, on the same substrate on one side of the LC, and may then generate fringe field(s). Fringe field switching may then occur, for example in a display device comprising electrodes of a plurality of neighbouring LC devices and/or LC elements.
  • Electrodes may further be provided electrodes on an opposite side of the LC.
  • in-plane electric fields may be created directly from both sides of the LC.
  • the electrodes may comprise interdigitated electrodes, for example within a layer on one substrate. Such interdigitated electrodes may be formed within, and separated by, an insulating layer on a substrate. Similarly, the electrodes may comprise finger- patterned electrodes on a layer and a plane electrode on another layer separated by an insulating layer on one substrate.
  • the above liquid crystal device configured such that an optic axis of the chiral nematic liquid crystal rotates in a plane normal to the local electric field component when the electric field is applied.
  • This may be achievable by the electrodes being arranged appropriately relative to the LC optic axis in the absence of electric field (see Fig. 1).
  • the optical axis of the chiral nematic LC is then preferably oriented in the plane of the electrodes when the electric field is applied.
  • the rotation is to align the optic axis to or towards the electric field component.
  • the plane normal to the local electric field component may be normal to the local direction of the electric field and/or may be perpendicular to the plane of the electrodes.
  • the above liquid crystal device configured to substantially fully align the liquid crystal molecule director in a direction substantially normal to the electric field component when said electric field is applied. This may apply across substantially all of the LC, or may apply to a region of LC to which the electric field component is local. Again, this may be achievable by the electrodes being arranged appropriately relative to the LC optic axis in the absence of e-field (see Fig. 1).
  • the director may be a local director at a point within the helical arrangement of molecules.
  • the full alignment of a local liquid crystal molecule director is to a direction substantially normal to the plane of the electrodes.
  • the above liquid crystal device wherein said at least two electrodes are configured to apply said electric field substantially fully normal to the helical axis of the chiral nematic liquid crystal, i.e., substantially fully perpendicular to the zero-field helical axis.
  • the electric field may be considered to be the electric field component.
  • the electric field may then be "in-plane".
  • substantially fully normal to the helical axis of the chiral nematic liquid crystal may relate to an electric field local to the helical arrangement or to an electric field applied uniformly over the entire LC.
  • the above applying of said electric field of “substantially fully normal” generally relates the moment of applying the field, i.e., the instant before which the helical arrangement responds by rotating.
  • the helical arrangement has a pitch such that transmission of said polarised light through said chiral nematic liquid crystal (LC) is substantially fully blocked in the absence of said electric field component, e.g., in the complete absence of electric field.
  • the pitch is short, i.e., shorter than the shortest wavelength of visible light, i.e., less than 380nm, and the liquid crystal may then be substantially isotropic.
  • the LC may have a hyper-twisted structure.
  • the pitch may be definable as the distance, parallel to the helical axis, between two points on the helix, where the orientation of the molecules has turned 360degrees).
  • the liquid crystal may be substantially isotropic as described above for example at normal incidence.
  • the pitch of the helical arrangement is preferably a short pitch.
  • any light referred to may be visible light, i.e., in the wavelength range of about 380nm to about 750nm including 380nnm and 750nm. This may for example apply where the device is the display device, light shutter or laser as described herein.
  • the light may be of the order to 1550nm, e.g., the optical communications C-band (1530 nm to 1565 nm), and/or may cover the optical communications L-band (1565 nm to 1625 nm).
  • the optical waveguide device, variable optical attenuator, optical switch and laser, which are described herein, may be used for telecommunications applications).
  • the above liquid crystal device wherein said substantially full blocking blocks at least about 95%, or preferably greater than about 98% or than about 99%, of the polarised light.
  • the blocked transmission is at least transmission parallel to the zero-field helical axis.
  • the device may, for example for display applications, block unpolarised light, for example where the device comprises polariser(s) to block a portion of the light having specific polarisation. (There may be present at least a polariser on the output side of the device, or there may be provided crossed polarisers as further described herein).
  • Blocking of transmission may similarly be about 95%, or preferably greater than about 98% or than about 99%. In embodiments that block non-polarised and/or polarised light, the degree of blocking may thus be lower at non-normal angles.
  • the pitch of the zero-field helical arrangement is less than 380nm (i.e., shorter than the shortest wavelength of visible light), preferably less than about 260nm, and more preferably less than about 150nm ("less than about 260nm" including 260nm).
  • the specific value of the pitch preferred for a given embodiment may be greater than or less than 380nm and/or may depend on the LC birefringence. Nevertheless, in any embodiment, the liquid crystal is preferably short pitch liquid crystal.
  • the chiral nematic liquid crystal has a thickness (along a direction parallel to the zero-field helical axis) such that said polarised light, which propagates through the LC parallel to the zero-field helical axis, is substantially fully transmitted though said chiral nematic liquid crystal in the presence of said electric field.
  • the device is a liquid crystal cell having a thickness of preferably less than about 20um, e.g., about 5um, and more preferably less than about 4.5um, e.g., 4.3um.
  • the above liquid crystal device configured to be operable by said application of said electric field to have a ratio, e.g., contrast ratio, of transmission of said polarised light in the presence of the electric field to transmission of said polarised light (at least parallel to the zero-field helical axis) in the absence of the electric field of greater than about 1000:1, preferably greater than about 6000:1, preferably at normal incidence.
  • the electrodes may be configured at an appropriate spacing to allow the device to be conveniently driven by a voltage sufficient to fully align the helix to the normal to the zero-field axis.
  • the above liquid crystal device configured (e.g., at least the electrodes are spaced and/or there is no breakdown voltage as above) to be operable by said application of said electric field to substantially fully align said helical arrangement to an electric field applied normally to the zero-field helical axis (i.e., to allow substantially full transmission of input light) in less than about 50ms, preferably less than about 1.5ms, more preferably less than about lms, even more preferably in a time of the order of 100s of microseconds, starting from a condition where there is no potential difference between the electrodes, i.e., a zero-field condition. Such a time is that during which the electric field is applied and maintained. Such maintenance is preferably to effect the switching to the transmissive state.
  • the above liquid crystal device configured (e.g., at least the electrodes are spaced and/or there is no breakdown voltage as above) to be operable by said removal of said applied electric field to substantially fully recover alignment of said helical arrangement in less than about 50ms, preferably less than about lOOus, starting from the transmissive state wherein the helical arrangement is substantially fully aligned to an electric field applied normally to the zero-field helical axis.
  • the above liquid crystal device further comprising: at least two polarisers each having a polarisation axis, wherein said two polarisers are crossed polarisers; and said chiral nematic liquid crystal is disposed between said crossed polarisers.
  • crossed polarisers an angle between the polarisation axes of the polarisers may be non-zero, i.e., the axes are non-aligned, preferably the angle being substantially 90 degrees. (Alternatively, the polarisers may be at an angle of about 45 degrees).
  • the planes of the polarisers themselves are preferably substantially parallel.
  • Such a device may further comprise a substrate, e.g. silicon, on which a said crossed polariser is disposed. For example, one polariser may be on the substrate if the cell has a stacked structure and there are only two polarisers.
  • an optic axis at each surface of said chiral nematic liquid crystal adjacent a said polariser is at an angle of substantially 45degrees to the polarisation axis of each said crossed polariser.
  • liquid crystal device wherein the liquid crystal comprises a chiral dopant such as BDH1281.
  • the above liquid crystal device further comprising inner substrates having unidirectionally rubbed polyimide alignment layers.
  • This may be particularly advantageous for achieving both a standing helix in the absence of a field and/or a planar- aligned nematic in the field 'on' state or in the field 'on' state.
  • the device comprises USH (Uniform Standing Helix) liquid crystal.
  • the above liquid crystal device further comprising a compensation plate, e.g., optical compensation film.
  • a compensation plate e.g., optical compensation film.
  • Such a plate may diffuse and/or phase retard light to widen the light output angle, e.g., viewing angle where the device is used for display.
  • the plate may - additionally or alternatively to being a compensation plate - be a diffusing plate.
  • the diffusing and/or compensation plate may diffuse and/or phase retard light.
  • the range of viewing angles over which the image will be of good quality is increased by use of such a plate, even in embodiments where the light comes out at substantially all angles, e.g., over a full 180degrees from a planar output surface of the device.
  • the liquid crystal device wherein the chiral nematic liquid crystal comprises dye such as dichroic dye, pleochroic fluorescent dye and/or a plurality of different coloured dyes.
  • the colours of the different coloured dyes may be red, yellow and blue, for example).
  • the dye may be absorptive or fluorescent dye.
  • a dye-guest effect may then be observed wherein the dye molecules reorientate with the above helix rotation, so that the dye effect is effectively switched on/off with the application of the electric field. In such an embodiment, there may be less advantage to providing input and/or output polarisers.
  • the liquid crystal device having a composition comprising said chiral nematic liquid crystal and polymer.
  • the liquid crystal device comprising at least one reflector, wherein said at least one reflector is preferably metallic, dielectric (e.g. a dielectric mirror), coloured, absorbing and/or fluorescent. (Coloured and absorbing reflectors selectively reflect colours/wavelengths). This may be advantageous where the light to be controlled is received on one side of the LC and reflected to be output from the same side, e.g., where the light is ambient light such as sunlight.
  • dielectric e.g. a dielectric mirror
  • coloured, absorbing and/or fluorescent Coldoured and absorbing reflectors selectively reflect colours/wavelengths.
  • a display device e.g. comprising a plurality of the above liquid crystal devices.
  • a display device may be, for example, an LCD display (preferably flat-panel) for a monitor, mobile phone, computer, television, etc.
  • an optical waveguide device comprising the above liquid crystal devices.
  • Such a waveguide device may be used for, e.g., optical computing, telecommunications or laser applications, e.g., a fibre-to-fibre interconnect.
  • a variable optical attenuator comprising the above liquid crystal device.
  • Such a VOA may be an optical attenuator operable by application of the electric field to control a degree of attenuation of polarised light, e.g., for amplitude modulation or equalisation of an optical telecommunications signal.
  • a laser comprising the liquid crystal device, wherein the chiral nematic liquid crystal comprises, e.g., is doped with, a light harvester such as laser dye (which may be added in solution, e.g., as a solution including laser dye molecules), fluorescent dye and/or quantum dots.
  • a light harvester such as laser dye (which may be added in solution, e.g., as a solution including laser dye molecules), fluorescent dye and/or quantum dots.
  • the dye may be attached to liquid crystal molecules, e.g. the light harvester may also comprise mesogenic moieties, chemically or synthetically attached to the light harvesting moiety, to promote solubility and ordering of the light harvesting moiety within the liquid crystal host.
  • an optical switch e.g., for use in a WDM system for blocking or passing WDM channels or single wavelength signals, or light shutter, comprising the above liquid crystal device.
  • a method of controlling outputting of light from a liquid crystal device comprising: applying an electric field to a helical arrangement of liquid crystal molecules of chiral nematic liquid crystal of said device; and said helical arrangement rotating to align the helical axis of the arrangement to said electric field, wherein said chiral nematic liquid crystal has negative dielectric anisotropy and said helical arrangement has helical pitch of less than 380nm.
  • the method may further comprise removing said electric field to return the helical axis orientation to the orientation that existed before said applying said electric field.
  • the device may further comprise at least one polariser, e.g., the helical arrangement of liquid crystal molecules may be provided between crossed polarisers.
  • a method of controlling transmission of polarised light comprising: applying an electric field across chiral nematic liquid crystal disposed between crossed polarisers, wherein the liquid crystal has negative dielectric anisotropy and a helical arrangement of liquid crystal molecules in the absence of an electric field, wherein said electric field has a component normal to the helical axis of the chiral nematic liquid crystal, i.e., normal to the zero-field helical axis.
  • a method may control transmission of unpolarised light.
  • the pitch of said helical arrangement is less than 380nm, preferably less than about 260nm, more preferably less than about 150nm.
  • the pitch of the helical arrangement is again shorter than the shortest wavelength of visible light.
  • the specific value of the pitch preferred for a given embodiment may be greater than or less than 380nm and/or may depend on the LC birefringence.
  • the liquid crystal is preferably short pitch liquid crystal.
  • the electric field is applied such that the liquid crystal molecules have a helical arrangement, the helical axis of which is aligned to said electric field when the electric field is applied and preferably maintained.
  • a helical arrangement is retained and the helical axis rotates towards alignment with the applied electric field to put the device in a transmissive state, the electric field being local to the rotated helical arrangements of molecules.
  • the electric field is applied such that the optical axis of the chiral nematic LC aligns to be in the plane of the electrodes and parallel to the electric field. More specifically, the electric field is applied such that the optical axis of the chiral nematic LC aligns to be substantially parallel to the plane of the electrodes and/or substantially parallel to the electric field.
  • the electric field is applied such that an optic axis of the chiral nematic liquid crystal rotates normal to the electric field component.
  • the applied electric field is the electric field component, i.e., is fully normal local to the zero-field helical axis.
  • the rotation is to align the optic axis to or towards the electric field component.
  • the plane normal to the local electric field component may be normal to the local direction of the electric field and/or may be perpendicular to the plane of the electrodes.
  • the above method comprising applying said electric field component continuously throughout a time period of less than about 50ms, preferably equal to or less than about 1.5ms, more preferably equal to or less than about lms, to substantially fully align said helical arrangement to said electric field, preferably starting from a zero-field condition.
  • the above method comprising removing said electric field component continuously throughout a time period of less than about 50ms, preferably equal to or less than about lOOus, more preferably equal to or less than about 35us, to substantially fully recover alignment of said helical arrangement, preferably starting from the transmissive state wherein the helical arrangement is substantially fully aligned to an electric field applied normally to the zero-field helical axis.
  • the recovered alignment is to a direction of the zero-field helical axis.
  • the LC may have returned to the helical structure that it would have in the permanent absence of an electric field.
  • the above method comprising said applying of said electric field according to a predetermined transmission greyscale.
  • the strength of the electric field may be continuously varied to achieve analogue variation of degree of transmission between fully dark and fully transmissive states.
  • a liquid crystal device for controlling transmission of polarised light, comprising: chiral nematic liquid crystal having a helical arrangement of liquid crystal molecules in the absence of an electric field; and at least two electrodes for applying an electric field having a component normal to the helical axis of the chiral nematic liquid crystal, wherein the chiral nematic liquid crystal has a negative dielectric constant such that an optic axis of the chiral nematic liquid crystal rotates in a plane normal to the electric field component when the electric field is applied.
  • the optic axis of the chiral nematic liquid crystal rotates to align to the electric field component when the electric field is applied.
  • similar arrangements may differ in that the LC has negative dielectric anisotropy additionally or alternatively to the negative dielectric constant).
  • a liquid crystal device for controlling transmission of light, comprising: a light source to emit said light; chiral nematic liquid crystal having a helical arrangement of liquid crystal molecules in the absence of an electric field; and at least two electrodes for applying an electric field having a component normal to the helical axis of the chiral nematic liquid crystal, wherein the chiral nematic liquid crystal has negative dielectric anisotropy and is liquid crystal having pitch shorter than a shortest wavelength of said light.
  • the LC is USH (generally, USH is an arrangement in the absence of the electric field), the device electrodes are in-plane electrodes, and/or the device can be used as an intensity modulator between crossed polarisers.
  • the controlled light may be polarised or unpolarised light.
  • the following aspects generally concern embodiments using LC having positive dielectric anisotropy.
  • all references to short pitch may be taken as meaning having pitch shorter, preferably substantially shorter, than the wavelength of the controlled light or, more specifically, shorter than visible wavelengths, i.e., less than 380nm.
  • the device may have a light source for sourcing the light to be controlled.
  • the device may be for controlling polarised and/or unpolarised light).
  • a liquid crystal device for controlling outputting of light from said device, the device comprising: chiral nematic liquid crystal having a helical arrangement of liquid crystal molecules and having positive dielectric anisotropy; at least two electrodes for applying an electric field having a component normal to the helical axis of the chiral nematic liquid crystal molecules, the chiral nematic preferably having pitch shorter than a shortest wavelength of said light; the liquid crystal such that the helical arrangement of molecules rotates towards alignment with the electric field, preferably to align with the local electric field, wherein the liquid crystal is provided in a composition further comprising polymer.
  • Provision of the polymer is to advantageously stabilise the helical arrangement of the liquid crystal, an advantage thereof being to reduce a switching time of the device.
  • the polymer may be in the form of monoacrylate or diacrylate.
  • the helical arrangement in the absence of the electric field comprises a standing helical arrangement, i.e., is not ULH (Uniform Lying Helix), e.g., may be USH (Uniform Standing Helix).
  • the light may or may not be polarised.
  • the chiral nematic liquid crystal molecules may be helically arranged in the presence of said electric field, a helical axis of said arrangement in said presence of said field being aligned to said electric field applied to said molecules.
  • the liquid crystal helical arrangement may be to dielectrically couple to the electric field to rotate the helical axis of said helical arrangement in a direction dependent on the direction of the electric field.
  • the device may be configured such that an optic axis of the chiral nematic liquid crystal rotates in a plane normal to the electric field component when the electric field is applied, the rotation preferably to align the optic axis to the electric field.
  • the at least two electrodes may be configured to apply said electric field substantially fully normal to the helical axis of the chiral nematic liquid crystal.
  • the helical arrangement may have a pitch such that transmission of said light through said chiral nematic liquid crystal is substantially fully blocked in the absence of said electric field component, preferably to block at least about 95% of the light.
  • the LC may be less than 380nm, preferably less than about 260nm, more preferably less than about 150nm.
  • the chiral nematic liquid crystal may have a thickness such that said light is substantially fully transmitted though said chiral nematic liquid crystal in the presence of said electric field.
  • the device may be configured to be operable by said application of said electric field to have a ratio of transmission of said light in the presence of the electric field to transmission of said light in the absence of the electric field of greater than about 1000:1, preferably greater than about 6000:1.
  • the device may be operable by said application of said electric field to substantially fully align said helical arrangement to said electric field component in less than about 50ms, preferably less than about lms.
  • the device may be configured to be operable by removal of said applied electric field to substantially fully recover alignment of said helical arrangement in less than about 50ms, preferably less than about lOOus.
  • the device may further comprise: at least two polarisers each having a polarisation axis, wherein said two polarisers are crossed polarisers; and said chiral nematic liquid crystal is disposed between said crossed polarisers.
  • the liquid crystal may be comprised in a composition having polymer for stabilisation of molecular arrangements of the liquid crystal, preferably to reduce a switching response time of the device.
  • the polymer may be in the form of monoacrylate or diacrylate
  • the chiral nematic liquid crystal may comprise dye such as dichroic dye, pleochroic fluorescent dye and/or a plurality of different coloured dyes.
  • the device may have the chiral nematic liquid crystal comprised in a composition further comprising polymer.
  • the device may comprise at least one reflector, wherein said at least one reflector is preferably metallic, dielectric, colour, absorbing and/or fluorescent.
  • the at least two electrodes may be in a substantially common plane.
  • a display device comprising a plurality of the liquid crystal devices, an optical waveguide device comprising the liquid crystal device, a variable optical attenuator comprising the liquid crystal device, an optical switch comprising the liquid crystal device, a light shutter comprising the liquid crystal device, or a laser comprising the liquid crystal device wherein the chiral nematic liquid crystal comprises light harvester such as laser dye, fluorescent dye and/or quantum dots.
  • the device may be of any one of the device aspects using positive dielectric anisotropy as described herein.
  • a method of controlling output of light from a liquid crystal device comprising chiral nematic liquid crystal having a helical arrangement of liquid crystal molecules and having positive dielectric anisotropy and further comprising at least two electrodes for applying an electric field normal to the helical axis of the chiral nematic liquid crystal molecules, the liquid crystal provided in a composition further comprising polymer, the method comprising: applying the electric field; and rotating the helical arrangement towards alignment with the electric field, preferably to align with the electric field.
  • the helical arrangement in the absence of the electric field is not ULH, e.g., may be USH.
  • ULH e.g., may be USH.
  • a method of controlling output of light from a liquid crystal device comprising chiral nematic liquid crystal having a helical arrangement of liquid crystal molecules and having positive dielectric anisotropy and further comprising at least two electrodes for applying an electric field normal to the helical axis of the chiral nematic liquid crystal molecules, wherein: in the absence of the electric field, the orientation of the helical arrangement and optic axis of the chiral liquid crystal is such that the polarisation state of any linearly polarised light incident on the device is perpendicular to the optic axis and helical arrangement, and the liquid crystal is comprised in a composition having polymer, the polymer preferably being to a concentration of between about 0.1% and about 30% w/w in the host chiral liquid crystal, the method comprising: applying the electric field to rotate the helical arrangement and
  • the orientation of the helical arrangement and optic axis of the chiral liquid crystal is such that the polarisation state of any linearly polarised light incident on the device is perpendicular to the optic axis and helical arrangement, i.e., the method preferably does not use a ULH LC device, e.g., may use a USH LC device.
  • the alignment, or partial alignment, to a plane defined by the electrodes may be to a plane substantially parallel to the plane of the electrodes.
  • a liquid crystal device for controlling output of light from the device, the device comprising chiral nematic liquid crystal having a helical arrangement of liquid crystal molecules and having positive dielectric anisotropy and further comprising at least two electrodes for applying an electric field normal to the helical axis of the chiral nematic liquid crystal molecules, the device comprising: the liquid crystal such that, in the absence of the electric field, the orientation of the helical arrangement and optic axis of the chiral liquid crystal is such that the polarisation state of any linearly polarised light incident on the device is perpendicular to the optic axis and helical arrangement, and the liquid crystal comprised in a composition having polymer, the preferably polymer being to a concentration of between about 0.1% and about 30% w/w in the host chiral liquid crystal; the liquid crystal such that application of the electric field rotate
  • a method of controlling output of light from a liquid crystal device comprising chiral nematic liquid crystal having a helical arrangement of liquid crystal molecules and having positive dielectric anisotropy and further comprising at least two electrodes for applying an electric field normal to the helical axis of the chiral nematic liquid crystal molecules, the method comprising: applying the electric field to rotate the helical arrangement and optical axis of the chiral nematic liquid crystal to align, or partially align, in a plane defined by the electrodes; after removal of the electric field, the optical axis and helical arrangement remaining aligned, or partially aligned, in the plane defined by the electrodes; and at least one further electrode applying a further electric field the at least one further electrode oriented to apply said further electric field substantially normal to the rotated axis of the helical arrangement and rotated optical
  • the rotated optical axis may be described as an induced optical axis.
  • the liquid crystal is such that, in the absence of the electric field, the orientation of the helical arrangement and optic axis of the chiral liquid crystal is such that the polarisation state of any linearly polarised light incident on the device is perpendicular to the optic axis and helical arrangement, i.e., the method preferably does not use a ULH LC device, e.g., may use a USH LC device.
  • the alignment, or partial alignment, to a plane defined by the electrodes may be to a plane substantially parallel to the plane of the electrodes.
  • Fig. 1 shows a schematic of a device according to an embodiment, including an illustration of the principle of operation
  • Fig. 2a shows experimental results of the transmission of the device as a function of the applied electric field
  • Fig. 2b shows the optical response demonstrating the rise and decay times of the device
  • Fig. 3 shows photographs of a cell of the device mounted on a light box for different electric field strengths
  • Fig. 4a shows isocontrast curves for the device with a compensation plate
  • Fig. 4b shows isocontrast curves for the device without a compensation plate
  • Figs. 5a - c show CIE diagrams of the device
  • Fig. 5d shows a colour contour plot for the device
  • Fig. 6 shows photomicrographs of N*LC of another embodiment that has a structure as in Fig. 1;
  • Fig. 7 shows electro-optic characteristics of the other embodiment
  • Fig. 8 shows the data for the all-electrical induced ULH device versus conventional (manual) induction
  • Fig. 9 shows a schematic of the ULH device
  • Fig. 10 shows a transmission- voltage characteristic of a polymer stabilized short-pitch LC device
  • Fig. 11 shows an arrangement wherein short pitch helical liquid crystal unwinds as further described herein.
  • Chiral nematic displays may use conventional electrodes or in-plane electrodes. Such devices will typically be based on the effect of selective reflection within the range of operational wavelengths. Such devices may be generated using short-pitch chiral nematics, such that the range of selective reflection lies below the range of operational wavelengths.
  • the chiral nematic may be aligned with the helical axis perpendicular to the plane of the device: variously called the 'standing-helix', 'planar aligned', or 'Grandjean' configuration.
  • the structure is effectively optically isotropic at normal incidence, leading to a dark state between crossed polarisers.
  • focal conic defects above the electrode areas may occur due to the non-uniform electric field distribution close to the electrodes.
  • Fig. 1 shows a schematic of the operating principle of the device: (a) with no field applied the chiral nematic liquid crystal is optically isotropic between crossed polarisers and the device is 'off ; and (b) an applied, in-plane, electric field causes the helical axis to lie in the plane of the device, resulting in a transmissive, 'on' state.
  • Fig. 1 shows a schematic of the operating principle of the device: (a) with no field applied the chiral nematic liquid crystal is optically isotropic between crossed polarisers and the device is 'off ; and (b) an applied, in-plane, electric field causes the helical axis to lie in the plane of the device, resulting in a transmissive, 'on' state.
  • FIG. 6 shows photomicrographs of the N*LC with a negative dielectric anisotropy and pitch 370 nm with: (a) no field applied, and under the application of an in-plane electric field of 400 V pp and frequency; and (b) 30 Hz, (c) 1 kHz , (d) 1 kHz with 15° cell rotation, (e) 1 kHz with 25° cell rotation and (f) 1 kHz with 45° cell rotation.
  • Fig. 3 shows photographs of the 4.3 micron cell between crossed polarisers on a light box, for six different electric field strengths.
  • the electro-optic cells are approximately lcm x lcm in dimensions and the polarisers cover the whole field of view.
  • Fig. 7 shows electro-optic characteristics of the device at 1 kHz between crossed polarisers.
  • the N*LC of Fig. 6 may be described as cholesteric/chiral nematic liquid crystal (note: cholesteric and chiral nematic can be used interchangeably).
  • the birefringence of the LC will cause light to be transmitted.
  • Samples were prepared by mixing a low concentration by weight of a high twisting power chiral dopant (BDH1305, Merck KGaA, helical twisting power 60 ⁇ m "1 ) into a nematic liquid crystal with a negative dielectric anisotropy ( ⁇ ⁇ -4) and birefringence of An ⁇ 0.07 (in-house mixture), using a precision balance (Mettler Toledo).
  • BDH1305, Merck KGaA Merck KGaA, helical twisting power 60 ⁇ m "1
  • ⁇ ⁇ -4 negative dielectric anisotropy
  • the sample was placed in a bake oven at a temperature of 100 0 C for a period of 24 hours to ensure sufficient mixing of the constituents via thermal diffusion.
  • ITO indium tin oxide
  • the electrode spacing and width was 15 ⁇ m and 5 ⁇ m, respectively.
  • a dopant may provide twisted or hyper- twisted LC.
  • an optically neutral to optically active switch such as negative dielectric anisotropy embodiments described herein, as well as positive dielectric-anisotropy-with-polymer system embodiments described later in this specification
  • they can be described as highly twisted, since this may be advantageous for the layer to be optically neutral to visible light wavelengths at zero field.
  • Photomicrographs, shown in Fig. 6, indicate that a lying-helix configuration is indeed obtained in the system. Further, the cell is seen to be free of disruptive defect structures. For clarity, a pitch of 370 nm is initially used, which has a considerable amount of transmittance in the field-off state (Fig 6a). This allowed for a comparison between the regions above, and in-between, the electrodes as the angle of the LC was rotated with respect to crossed polarisers. Under the application of an electric field, a uniform texture is observed between the electrodes (Fig. 6b-f). Above the electrodes, the standing-helix structure is seen to remain largely unchanged.
  • Photographs of the test-cell between crossed polarisers, and mounted on a light box, are presented for different electric field strengths. In the absence of an electric field, the cell is optically black, and there is no discernible difference between the cell and the background regions of only crossed polarisers. As the field strength is increased, the transmission increases in a controlled, smooth, way.
  • the measured transmission of the device is shown in Fig. 7a.
  • the change in transmittance through the device increases as the strength of the field is increased, with a threshold at approximately 5 V ⁇ m "1 .
  • the transmittance saturates.
  • the response time of the switching mechanism upon application and removal of the electric field is shown in Fig. 7b.
  • the switch-on and switch-off times, ⁇ on and ⁇ off were measured from the 10-90% and 90-10% transmission levels respectively.
  • ⁇ on 0.035 ms
  • ⁇ off 1.5 ms.
  • the mid-range grey-level to grey-level response is of the order -0.1 ms.
  • the contrast ratio of the device was found by measuring the ratio of the luminance in the field-on transmissive state (lying helix), to that in the field-off 'dark' state (standing helix).
  • the luminance was determined from the transmitted spectra by integrating the intensity as a function of wavelength from 380 nm to 780 nm, weighted by the standard colour matching function of the green component of light [E. Lueder, Liquid Crystal Displays: Addressing Schemes and Electro-Optical Effects (Wiley, 2001), p.137; J. Schanda, Colorimetry: Understanding the CIE System, (Wiley, Hoboken, 2007)].
  • the CR may be expected to be considerably higher if higher-quality polarisers were used, and/or the half-waveplate condition were optimised.
  • the transmitted intensity in the 'off state depends strongly on the pitch (approximately P 6 ), therefore a shorter pitch can lead to a contrast ratio that is considerably higher still.
  • the embodiment provides a liquid crystal display mode with response time ⁇ 1.5ms, and a contrast ratio of -1000:1. Further development of the materials and improvements to the device architecture, so as make ⁇ e values more strongly negative, and to ensure maximum field strength at the sample, will help to reduce the applied voltage required for switching. The results show that this switching mode has considerable potential for fast light shutters and flat-panel display modes.
  • the following describes an embodiment of a liquid crystal device in the form of a liquid crystal cell.
  • the device may advantageously be used to provide a high contrast liquid crystal display mode with, for example, a 100 microsecond response time. It may be particularly applicable to high definition flat panel television screens, for example as large as 100 inches.
  • a switching mode of the embodiment is based upon a chiral nematic liquid crystal that has a negative dielectric anisotropy, in-plane electrodes, and a hyper-twisted structure.
  • 'In-plane' generally means parallel to a plane defined by a substrate and/or polarisers of the device.
  • the LC appears optically black between crossed polariser as a result of the very short pitch ( ⁇ 150 nm) of the helical structure.
  • the short-pitch has further ramifications in that the time for the LC to relax to the field-off state is very fast (of the order of, or less than, ms).
  • Theoretical and experimental results show very high contrast combined with greyscale controllability.
  • the embodiment is applicable to liquid crystal displays (LCD), which involve high definition images with refresh rates above 100 Hz.
  • the response time of the LC component of the embodiment may be substantially unrestricted by the intrinsic visco-elastic driven relaxation of nematic LC.
  • a fast-modulating polarisation controller may be constructed provided the pitch of the helical structure was considerably less than the wavelength of the incident radiation.
  • the combination of an in-plane electric field and a standing helix geometry may result in a fast out-of-plane rotation of the optic axis as a result of a deformation of the helix due to dielectric and/or flexoelectric coupling.
  • the chiral nematic device is optically isotropic between crossed polarisers and no light is transmitted.
  • such a controller may be applicable for telecommunication applications whereby the wavelength of the incident radiation is of the order of 1550 nm, and this liquid crystal mode may further be applicable for a display application.
  • this liquid crystal mode may be advantageous. Bimesogenic materials may meet these criteria.
  • dielectric and/or flexoelectric coupling refer generally to coupling with the applied field, preferably to dieelectric coupling, which may optionally be combined with flexoelectric coupling though more preferably flexoelectric coefficients ar minimised).
  • the above embodiment may allow a fast-switching mode based upon a negative dielectric anisotropy chiral nematic and in-plane electrodes coated onto the inner surface of one of the substrates.
  • Samples were prepared by dispersing a low concentration by weight of a high twisting power chiral dopant (BDH1281, Merck KGaA) into a nematic liquid crystal with a negative dielectric anisotropy (Merck KGaA). These compounds were used as received and no further purification was carried out. After mixing on a precision balance (Mettler Toledo), the sample was then placed in a bake oven for a period of 24 hours to ensure sufficient mixing of the constituents via thermal diffusion.
  • BDH1281 high twisting power chiral dopant
  • Merck KGaA a high twisting power chiral dopant
  • a negative dielectric anisotropy Merck KGaA
  • the resultant mixture was injected into a 4.3 micron cell which had unidirectionally rubbed polyimide alignment layers on the inner substrates in order to achieve both a standing helix in the absence of a field and a planar- aligned nematic in the field 'on' state.
  • ITO indium tin oxide
  • FIG. 11 shows the low negative dielectric anisotropy whereas the low negative dielectric anisotropy implies that the coupling between the field and the LC is quite small.
  • the response is very short and is evident both from the rise and the decay times.
  • Figure 2b shows the optical response of the LC, plotted on the secondary axis, to a square wave with electric fields from 0 V/um to 18 V/um which is plotted on the primary axis. From this, the rise and decay times are found to be 1 ms and 100 us, respectively.
  • the rise time is dependent upon the field strength whereas the decay time is found to be independent of the field strength.
  • the rise time may be short by virtue of the fact that the dielectric coupling is quadratic in the field.
  • the short decay time is due to the very short pitch of the helix. Using hydrodynamic considerations it is possible to show that the response is quadratic in the pitch.
  • Photographs of a 4.3 micron-thick cell with in-plane electrodes between crossed polarisers and mounted on a light box are presented for different electric field strengths in Figure 3. It can be seen that the cell is optically black and there is no discernible difference between the cell and the regions of crossed polarisers only. As the field strength is increased the sample becomes more and more transmissive as the helical structure rotates. There is a small amount of transmission at 3.3 V/um close to the threshold and increases dramatically until it reaches saturation. The maximum brightness is shown at a field strength of 16.7 V/um.
  • the device may thus advantageously combine grey scale with short response times.
  • FIG. 5a CIE diagrams are shown in Figure 5. Three diagrams are shown corresponding to different polar and azimuthal angles and one diagram showing the colour contour. Each one was obtained using the Berreman 4 x 4 matrix and Standard Illuminant C. Here we assume that the field is applied linearly and uniformly and that there is no degree of chirality present. It is also assumed that there is no pretilt of the molecules at the surfaces of the substrates at both the light source and observer side. The first diagram (Fig. 5a) is for a fixed polar angle of 50° and the azimuthal angle is then varied from 0 to 360°.
  • Figures 5b and 5c are for fixed azimuthal angles of 0 and 45°, respectively, and the polar angle is varied from 0 to 80°.
  • An azimuthal angle of 0° corresponds to the polariser direction whereas an azimuthal angle of 45° corresponds to the optical axis in the field 'on' state.
  • Fig. 5c shows that there is a slight change in the chromaticity as the polar angle varies when the azimuthal angle is fixed at 45°.
  • the variation in chromaticity is very small except for some slight 'yellowing' at the extremes.
  • the embodiment may advantageously demonstrate a fast- switching liquid crystal display mode with response times of 100 us and contrast ratios of at least -1000:1 and even as high as 65000:1 at normal incidence. Due to a continual reorientation of the LC molecules, transmission voltage curves show that the response may advantageously allow for greyscale controllability.
  • the following describes a device and other arrangements and related methods, which use liquid crystal having positive dielectric anisotropy. (Though other arrangements may differ merely by substituting the positive anisotropic LC for zero dielectric anisotropy LC).
  • the LC is provided in a composition further comprising a polymer (for example by adding reactive mesogen) as may be the case in any positive dielectric device/arrangement/method described herein, the polymer may be in the form of monoacrylate or diacrylate, this relating to the end groups that cross-link.
  • the helical arrangement of molecules rotates to align to the local electric field, i.e., the field directly influencing the orientation of the molecules.
  • the at least two electrodes are in a substantially common plane, e.g., are in-plane electrodes.
  • a related method is of controlling output of light from a liquid crystal device, the device comprising chiral nematic liquid crystal having a helical arrangement of liquid crystal molecules and having positive dielectric anisotropy and further comprising at least two electrodes for applying an electric field normal to the helical axis of the chiral nematic liquid crystal molecules, the method comprising: applying the electric field; and rotating the helical arrangement to align with the electric field.
  • the helical arrangement may rotate to align to the local electric field, i.e., the field existing locally to the molecules of the helical arrangement or which may be uniform over the entire LC.
  • the helical arrangement that exists in the absence of any electric field is referred to as a zero- field arrangement.
  • the electric field local to the helical arrangement is normal to the orientation of the helical axis of the zero-field arrangement.
  • the light outputted by the device may be received by the device from an external source (e.g., sunlight, an external fluorescent source, LED, etc.) or may be generated internally, e.g., where light emitters are added to the LC to form, e.g., a laser.
  • an external source e.g., sunlight, an external fluorescent source, LED, etc.
  • the above control may be of the proportion of generated light that forms the output light, and/or of the direction of transmission of the output light.
  • the helical arrangement of the chiral nematic (i.e., cholesteric) LC may rotate to become aligned with the electric field.
  • the optical axis of the chiral nematic LC may reorient to align in the plane of the electrodes when the electric field is applied.
  • the alignment may be full, for example where the device is used as a binary device.
  • the LC may be controlled to rotate towards partial, i.e., incomplete, alignment with the electric field, depending for example on the strength of the electric field.
  • the rotation may occur without the LC helical arrangement unwinding.
  • the device may be operable by application of the electric field to substantially fully align the helical arrangement with the electric field in less than about 50ms, preferably less than about 10ms, preferably less than about lms. (Other arrangements may differ from the or each of the aspects and embodiments described herein by the zero-field helical arrangement unwinding as shown in Fig. 11, alternatively or additionally to the rotation to align to the electric field).
  • the pitch of the helical arrangement may be short or long.
  • a short pitch is a pitch that is shorter than the wavelength of visible light.
  • the pitch is less than 380nm, more preferably less than about 260nm, e.g., about 150nm.
  • the specific value of the pitch preferred for a given positive dielectric anisotropy embodiment may be greater than or less than 380nm and/or may depend on the LC birefringence.
  • the liquid crystal is preferably short pitch liquid crystal.
  • the pitch may be such that the transmission of light through the LC is at least partially, preferably substantially fully, blocked in the absence of the electric field.
  • Such an embodiment may comprise a polariser at least on a light input side of the device.
  • the device may comprise polarisers on opposite sides of the LC, these polarisers having substantially perpendicular transmission axes.
  • the blocking may be of at least about 95%, preferably about 100%, of polarised light incident on the input side of the LC.
  • the LC preferably substantially fully transmits the light., e.g., transmits at least about 95% of the light.
  • the liquid crystal device may comprise at least two polarisers each having a polarisation axis (i.e., transmission axis), the polarisers comprising a pair that are aligned such that their axes are substantially perpendicular to each other (i.e. the polarisers are crossed), the chiral nematic LC being disposed between these two polarisers.
  • the pair may have their axes substantially parallel to each other.
  • crossed polarisers is preferable, and in this case, the optical axis of the LC in the absence of the electric field may be at an angle of substantially 45 degrees to the polarisation axis of each said crossed polariser).
  • the polarisers may be disposed on respective substrates of the device.
  • the device may be operable by the application of the electric field to have a ratio of transmission of the light (preferably polarised; internally or externally generated) in the presence of the electric field to transmission of the polarised light in the absence of the electric field of at least about 1000:1, preferably higher, e.g., at least about 6000:1.
  • the light controlled by the device may be polarised light, for example light that is input into the device from a polariser on one side of the LC.
  • the LC helical arrangement which may be short- or long-pitched, is stabilised by polymer.
  • the liquid crystal may be comprised in a polymer composition, the polymer advantageously providing some elasticity to the LC.
  • Such elasticity may make the LC more rugged, e.g., less susceptible to permanent damage when the LC is compressed, e.g., by pressing by a device user's finger.
  • the elasticity may reduce hysteresis in the switching characteristic of the LC device, so that the switching time (i.e., time for alignment/de- alignment) is changed.
  • the time of de-alignment of the LC i.e., for rotation of the LC helix to return to its original orientation, i.e., the orientation before the electric field was applied
  • the time of de-alignment of the LC is reduced due to the spring-like action of the polymer.
  • the LC advantageously comprises dual frequency chiral nematic LC.
  • the dielectric anisotropy changes sign with the frequency of the applied field. This may be advantageous to ensure that the LC helical arrangement rotates to return to the original position when the electric field is removed, i.e., rotates reversibly.
  • the dual frequency LC may for example have a negative dielectric anisotropy within a frequency range, e.g., 100Hz - IkHz, and positive dielectric in another frequency range, e.g., above IkHz.
  • the frequency at which the dielectric anisotropy changes sign is less than about 100kHz.
  • the alignment of the LC helical axis may be recovered by application of a further electric field in a different frequency range.
  • Dual frequency chiral nematic LC may be advantageous for example where the LC is not polymer-stabilised.
  • the LC may be comprised in a composition having a low concentration of reactive mesogen (e.g., Merck RM-257) or polymer, for example as described herein in relation to embodiments using LC having negative dielectric anisotropy).
  • the LC device may nevertheless be advantageous for, e.g., aligning a ULH (uniform lying helix), since such alignment of the LC helical arrangement may be stable in zero electric field even after rotation has occurred to align the helix to an applied electric field.
  • a ULH comprising the liquid crystal device.
  • the above polymer composition may be achieved by adding to the LC a low concentration of polymer or of reactive mesogen (e.g., Merck RM257); which crosslinks to form polymer.
  • concentration of added mesogen or polymer is preferably ⁇ 20% w/w (weight by weight) relative to the LC.
  • the electrodes are configured to apply the electric field substantially fully normal to the zero-field helical axis of the LC.
  • the electrodes may comprise at least two electrodes on the same surface e.g., top or bottom surface, of the LC or on a surface of a substrate subsequently brought into direct or indirect contact with the LC.
  • Such sets of electrodes may be found on two or more respective surfaces, e.g., first and second sets on the lower and upper surfaces of the LC, respectively.
  • any such set of electrodes may advantageously be configured to generate a fringe field.
  • the electrodes for applying an electric field may comprise electrodes on opposite sides of the LC, e.g., on the top and bottom of the LC or on substrates adjacent respective opposite sides of the LC.
  • first and second sets of electrodes on respective surfaces may be provided to apply respective electric fields that each have a component normal to the helical axis.
  • the electrodes of the LC device may comprise interdigitated electrodes on one side of the LC, e.g., on a substrate attached directly or indirectly to an upper or lower surface of the LC.
  • the interdigitated electrodes may be separated from the substrate by an insulating layer.
  • the electrodes may comprise finger-patterned electrodes on a layer and a plane electrode on another layer separated by an insulating layer (e.g., barrier layer) in the substrate.
  • the LC of the device may comprise one or more dyes, which may be absorptive or reflective dyes. More specifically, the dye(s) may be dichroic dye, pleochroic fluorescent dye, and/or a plurality of different coloured dyes, e.g., red, yellow and blue.
  • the above chiral nematic LC rotation to align with the electric field may further cause the dye molecules to rotate.
  • this effect of the dyes within the device may effectively be switched on/off., by means of a dye-guest host effect.
  • the provision of the single or at least two polarisers as described above may then be less advantageous.
  • the LC device may be a reflective device.
  • external light e.g., sunlight
  • the LC device may comprise a reflective element that is, e.g., metallic, dielectric (e.g., a dielectric mirror), coloured, absorbing and/or fluorescent.
  • a coloured or absorbing reflective element may selectively reflect different colours/wavelengths.
  • the reflective LC device may be provided with a single, or no, polariser.
  • a display device comprising a plurality of the LC devices having positive dielectric anisotropy, which preferably include any combination of the above optional features.
  • a display device may comprise a optical compensation film, which may widen the viewing angle of the display device by dispersing or diffusing the light output from the LC devices.
  • the plate may - additionally or alternatively to being a compensation plate - be a diffusing plate.
  • the diffusing and/or compensation plate may diffuse and/or phase retard light.
  • the range of viewing angles over which the image will be of good quality is increased by use of such a plate, even in embodiments where the light comes out at substantially all angles, e.g., over a full 180degrees from a planar output surface of the device.
  • an optical waveguide device e.g., for optical computing, telecommunication or data communication, comprising at least one of the LC devices having positive dielectric anisotropy, the LC device preferably including any one or combination of the above optional features.
  • a variable optical attenuator an optical switch (e.g., for a wavelength division multiplexing (WDM) system and/or for blocking or passing WDM or single wavelength signals) or a light shutter, comprising at least one of the LC devices having positive dielectric anisotropy, the LC device preferably including any one or a combination of the above optional features.
  • WDM wavelength division multiplexing
  • a liquid crystal laser device comprising at least one LC device of the first arrangement, the LC device preferably including any one or combination of the above optional features.
  • the chiral nematic liquid crystal is doped with a light emitter or light harvester.
  • the emitter or harvester may be a laser dye (this may be provided in liquid form, e.g. laser dye molecules in a solution and/or may be chemically attached to LC molecule), rare earth element(s), fluorescent dye and/or quantum dots.
  • the laser device is for reorienting the direction and/or degree of transmission of a light beam output from the laser device.
  • the device may be a switchable liquid crystal element, either a display or phase modulation device, based upon the uniform lying helix (ULH) geometry in chiral nematic liquid crystals.
  • the ULH is an in-plane alignment of the chiral nematic liquid crystal helical axis.
  • the ULH has previously been aligned by a complex and non- systematic combination of mechanical shearing, temperature ramping and electric fields. Induction of the ULH therefore requires some individual expertise and does not lend itself to mass production.
  • a very surprising result has been the observation of the induction of the uniform lying helix (ULH) by in-plane electric fields. This technique also appears to offer increased stability of the induced texture e.g. the ULH is preserved on a much longer timescale without an external field applied. Traditionally seen in these experiments, the texture decays rapidly (order of seconds to minutes) to the Grandjean/focal conic state.
  • the electrical induction of the ULH is surprising, this observation may not always be sufficient to generate a switchable device (e.g.
  • phase modulator or display A further preferable feature is the incorporation of a third, plane-parallel electrode above the in-plane substrate.
  • the device is formed as follows: the ULH is induced by the in-plane electric field; the field then is removed and the in-plane electrodes shorted (same potential). The device is then addressed by applying a voltage between the upper electrode and the (now common) substrate electrodes.
  • the graph of Fig. 8 shows the data for the all-electrical induced ULH device versus conventional (manual) induction. Although the tilt angle is reduced, the material used in the investigation possess only modest flexoelectric and/or dielectric coupling, which may govern the response. Using new materials, designed for improved flexoelectric and/or dielectric coupling, switching voltages of the order of several V/ ⁇ m are possible for a typical cell thickness of 3 - 5 ⁇ m. A schematic of the device is shown in Fig. 9.
  • the device may include a reproducible and electrically automated induction of the ULH texture. Furthermore, the ULH device, once formed, may permit fast switching (down to lO ⁇ s), low voltage (conventional circuitry can be used), analogue (grey-scale) operation and/or phase modulation applications.
  • a short- pitch chiral nematic liquid crystal with polymer with in-plane electric fields for example for a fast- switching modulation device.
  • Such a device may be a display device using a polymer stabilised short pitch chiral nematic liquid crystal with a large positive dielectric anisotropy.
  • the display can be switched between an optically extinct Off state to a bright On' state by the action of an external electric field and return to the optically extinct state after removal of the applied field.
  • the device may possess excellent contrast and response times of the order of several milliseconds for On and sub-milliseconds for Off.
  • the graph of Fig. 10 shows the effect of polymer stabilising a short pitch material.
  • the material is addressed by an in-plane field and is positioned between crossed polarisers. In this device, the voltage is ramped from OV to 200V then back down to OV to investigate the hysteresis properties. From the graph, for the non-polymer stabilised material, the device does not recover to the original (unswitched) state and therefore cannot be used as a display device.
  • the material comprises a reactive liquid crystal compound and is suitably UV polymerised
  • the device formed allows excellent recovery of the original unswitched state with minimal hysteresis. This is a surprising result and allows a device to be formed with very good contrast and fast response times e.g. On time of 5ms and Off time of 0.2ms, or On time of 0.5ms and Off time of 0.2ms. In yet more advantageous embodiments, response times of the order of 100 us for on and off have been achieved.
  • Potential advantages of the device may include: short pitch material allows an optically extinct Off state which permits excellent contrast; positive dielectric coupling (allows use of existing materials to significantly lower operating voltages); in-plane switch technology; and/or fast response.

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  • Physics & Mathematics (AREA)
  • Nonlinear Science (AREA)
  • Chemical & Material Sciences (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • General Physics & Mathematics (AREA)
  • Optics & Photonics (AREA)
  • Mathematical Physics (AREA)
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  • Liquid Crystal (AREA)

Abstract

La présente invention concerne d'une manière générale un dispositif à cristaux liquides, et plus particulièrement un dispositif sous la forme d'une cellule à cristaux liquides destinée par exemple à un dispositif d'affichage. La présente invention concerne en outre un dispositif d'affichage comportant ledit dispositif à cristaux liquides, un dispositif à guide d'ondes optique comprenant ledit dispositif à cristaux liquides, un atténuateur optique variable doté dudit dispositif à cristaux liquides, un commutateur optique muni dudit dispositif à cristaux liquides, un procédé de contrôle de l'émission de lumière polarisée, ainsi qu'un autre dispositif à cristaux liquides. Un dispositif à cristaux liquides selon l'invention qui permet de contrôler l'émission de lumière polarisée comprend : un cristal liquide nématique chiral ayant un agencement hélicoïdal de molécules de cristaux liquides en l'absence d'un champ électrique ; et au moins deux électrodes servant à appliquer un champ électrique qui possède un constituant perpendiculaire à l'axe hélicoïdal du cristal liquide nématique chiral. Ce cristal liquide nématique chiral présente une anisotropie diélectrique négative.
PCT/GB2010/050929 2009-06-02 2010-06-02 Dispositif à cristaux liquides comprenant un matériau à cristaux liquides nématiques chiraux ayant un agencement hélicoïdal Ceased WO2010139995A1 (fr)

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CN2010800342348A CN102460290A (zh) 2009-06-02 2010-06-02 包括成螺旋布置的手性向列型液晶材料的液晶装置
US13/322,834 US20120140133A1 (en) 2009-06-02 2010-06-02 Liquid crystal device comprising chiral nematic liquid crystal material in a helical arrangement
EP10725843A EP2438485A1 (fr) 2009-06-02 2010-06-02 Dispositif à cristaux liquides comprenant un matériau à cristaux liquides nématiques chiraux ayant un agencement hélicoïdal

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GBGB0909422.8A GB0909422D0 (en) 2009-06-02 2009-06-02 Electro-Optical Switching Device
GB0918745.1 2009-10-26
GBGB0918745.1A GB0918745D0 (en) 2009-06-02 2009-10-26 Electro-optical switching device

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US11327385B2 (en) 2012-09-30 2022-05-10 Optica Amuka (A.A.) Ltd. Polarization-insensitive phase modulator
WO2017158486A1 (fr) * 2016-03-16 2017-09-21 Optica Amuka (A.A.) Ltd. Modulateur de phase insensible à la polarisation

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US20120140133A1 (en) 2012-06-07
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EP2438485A1 (fr) 2012-04-11
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